Three dimensional shear wave structure in the upper mantle beneath the oldest Pacific plate

The Pacific Array, which is a large-scale array experiment covering the entire Pacific Ocean by repeating ~two-year-long seafloor observations in various part of the ocean to reveal the physical nature of the lithosphere-asthenosphere system (LAS), has been launched in 2018.

As a first part of the Pacific Array, Japan-South Korea joint group conducted a one-year seafloor observation (Oldest-1 Array) with 12 broadband ocean bottom seismometers and 7 ocean bottom electro-magnetometers in 2018 on the oldest seafloor in the Pacific ocean, located 1000 km east of the Guam island, to reveal the evolution of the oldest part of the Pacific plate (~170 Ma). We measured the phase speed dispersions of Love and Rayleigh waves up to 4th higher mode with Oldest-1 Array data, as well as available broadband data in the Pacific Ocean to determine the three dimensional upper mantle shear wave structure beneath the whole Pacific Ocean. Oldest-1 Array data improve the shear wave structure beneath the oldest seafloor region of the Pacific Ocean. The obtained model suggests that large-scale anomalies are similar to our previous model (Isse et al., 2019, EPSL). However, beneath the Oldest-1 Array, we found that lithosphere is faster and thicker than those in the previous model. The shear wave speeds in the western Pacific region shows that the shear wave speeds at ages between 140-150Ma is the fastest and that seafloor-age dependence at ages older than 140 Ma is none or weak.